Fig. 1: Infrared LAS sensors integrated into a pulse-detonation combustor at the Naval Postgraduate School.

Fig. 1: Infrared LAS sensors integrated into a pulse-detonation combustor at the Naval Postgraduate School.

Laser-absorption spectroscopy (LAS) is a robust technique for providing quantitative, species-specific measurements of gas properties in a wide range of environments. By using state-of-the-art wavelength-modulation techniques and optical engineering, LAS sensors can be integrated into hostile systems (e.g., engines, power plants, etc.) for real-time feedback and control. The figure below shows time-resolved measurements of gas temperature and H2O mole fraction (left) used to calculate time-resolved enthalpy (right) in a pulse-detonation combustor.

In LAS, the wavelength of a laser is tuned across an absorption transition of a specific molecule and the transmitted light intensity is recorded on a photodetector (Fig. 3). The amount of light that is absorbed (known as the absorbance, see Fig. 4) is dictated by fundamental constants and the population of molecules in the absorbing state, the latter of which is set by the temperature and number density of the gas. By measuring the absorbance of two transitions with different lower-state energy, the temperature of the gas can be calculated. With the temperature, pressure, and path length through the gas known, the mole fraction of the absorbing species can be calculated from the absorbance of a single transition. The pressure and velocity of the gas can be inferred from the shape and location, respectively, of absorption transitions.

Fig 3: Optical transmission through a 10 cm path of 10% water in air at 1 atm.

Fig 3: Optical transmission through a 10 cm path of 10% water in air at 1 atm.

Fig. 4: Absorbance spectra of 10% water in air at 1 atm across a 10 cm path.

Fig. 4: Absorbance spectra of 10% water in air at 1 atm across a 10 cm path.

Fig. 5: Infrared absorption bands of a few common chemical species.

Fig. 5: Infrared absorption bands of a few common chemical species.

In the infrared, absorption transitions correspond to an increase in a molecule's rotational and vibrational energy. With the exception of homonuclear diatomics (e..g, O 2 ), all molecules posses a unique infrared absorption spectrum, commonly known as their spectral fingerprint. As a result, infrared laser-absorption sensors can provide measurements of a wide range of chemical species. The absorption bands of a few molecular species are shown in Fig. 5. Each absorption band correponds to a unique change in the vibrational energy of a given chemical bond(s) (e.g., C-H, O-H).

The Goldenstein Group is focused on developing next-generation LAS sensors with improved robustness, sensitivity, time-response, and dimensionality (spatially and/or chemically) for studying challenging environments and systems, in particular, propulsion systems and energetic-material combustion. As a result, research is ongoing in:

  1. Infrared spectroscopy
  2. Wavelength-modulation and hyperspectral diagnostic techniques
  3. Signal processing
  4. Optical engineering
  5. Combustion and gas dynamics
  6. Integration of sensors into engineering and scientific systems

using state-of-the-art lasers and optical equipment.